Composite materials such as carbon/carbon composite materials (CIC) have been known for more than thirty years for their excellent resistance to temperature and their resistance to thermal shocks. They retain their breaking stress and their modulus at temperatures of more than 2,000° C. However, within the scope of prolonged use in an oxidizing medium, they rapidly lose any mechanical property and this from 400° C. This is why, for overcoming this major problem, about twenty years ago, composites with an SiC matrix and with C fibers were first of all developed and then with SiC fibers when the latter were available on the market. However, if SiC/SiC or C/SiC composites have better resistance to oxidation than C/C composites, they cannot be used at temperatures above 1,200° C.
Protective coatings against oxidation of materials based on C, and notably C/C composite materials, may be of various chemical natures.
Notably, a distinction is made between coatings based on noble metals, coatings based on phosphates; coatings based on boron oxide, on borates or borides; coatings based on carbides.
We shall first of all examine coatings based on noble metals.
Among noble metals, iridium is of particular interest as a protection against oxidation (PAO), because of its high melting temperature (2,440° C.) and of its very low permeability to oxygen up to about 2,100° C.
During the 60's, iridium was particularly studied within the scope of the space program of the United States [1]. The main problem which is posed upon using iridium in order to achieve PAO, is the extreme volatility of iridium oxides (IrO2 and IrO3).
Further, the coefficient of expansion difference between iridium and carbon makes it difficult to obtain an adherent PAO.
However, dense iridium deposits adhering to carbon have been obtained.
Thus, document [2] first of all describes a method comprising the deposition of a slurry of finely divided iridium powder on a graphite substrate, and then the heating of the coated graphite substrate to a temperature above 2,130° C. and for a sufficient time in order to obtain a coating made of melted iridium containing recrystallized graphite.
Document [3] describes a method for making an iridium coating on a graphite substrate, wherein a slurry of iridium powder is deposited on a graphite substrate, and the graphite substrate is then heated in order to produce by sintering an agglomerated iridium coating on this graphite substrate, this coating for example having a thickness of about 60 μm. It is then proceeded with depositing iridium in the vapor phase on the agglomerated iridium coating, for example by sublimation of carbonyl or chlorocarbonyl iridium in a carbon monoxide atmosphere under reduced pressure between 300° C. and 800° C. The thickness of the thereby achieved iridium deposit is for example of about 15 μm. Finally deposition by electro-plating of an iridium layer on the iridium deposited in the vapor phase is achieved.
It is possible to estimate the thickness of the final deposit as being about 100 μm.
The PAO prepared in document [3] was tested and it resists to oxidation in air, for example up to 2,000° C., for 10 minutes, without any degradation.
Moreover, according to document [1], the recession rate of iridium in air at 2,000° C. at atmospheric pressure is of about 150 μm/hour.
As a conclusion, the drawbacks of PAOs based on noble metals like iridium are mainly the cost, and the formation of volatile species, such as IrO2 and IrO3, which are highly unstable.
Another category of protective coatings against oxidation consists of coatings based on phosphates.
Enamels based on phosphates are often described in the literature, and notably in document [1], for protection of carbonaceous materials.
Document [4] describes a method for achieving protection against the oxidation of a product made of a composite material comprising carbon and having a ceramic surface wherein an external coating with a composition based on P2O5—SiO2—Al2O3 is formed on the ceramic surface, and after drying, a heat treatment is performed at a temperature at least sufficient for transforming the external coating into an insoluble cement capable of forming a self-healing glass.
This type of protection based on phosphates is according to document [1], effective for temperatures ranging up to 1,000° C.
The main drawback of PAOs based on phosphates is that they only ensure protection at temperatures below 1,200° C.
Other PAO coatings are coatings based on boron oxide and on borates or borides.
Coatings based on zirconium boride (ZrB2) are, according to document [1], effective for protecting over a very short time, graphite up to 2,200° C.
Boron oxide may also be used for ensuring cohesion of refractory particles such as ZrB2, HfB2 or ZrSi2 in order to form an impervious coating at the surface of the material. After 10 hours in air at 1,200° C., a C/C composite coated with such a protection is, according to document [5], always entirely intact.
The main drawback of PAOs based on borates, borides, or boron oxides is that there is always formation of B2O3 which volatilizes from 1,200° C.
PAO coatings may also be based on carbides.
Silicon carbide (SiC) is very interesting for producing PAOs resistant to high temperatures, for example above 1,200° C., since it forms an oxide with the oxygen of the air, which, beyond 1300° C., has the lowest permeability to oxygen of all the oxides [1].
Further, under certain conditions of temperature and of O2 partial pressure, SiC forms a glassy phase which gives the possibility, by topping the carbon, of blocking the pores and the possible cracks.
This PAO seems to be effective between 1,000° C. and 1,800° C. [1] depending on the oxygen partial pressure.
Chemical vapor deposition (CVD) is the means the most currently used for depositing SiC with a view to producing a PAO [1].
Document [6] describes parts made of refractory materials prepared by hot pressing of TiB2, ZrB2, HfB2, NbB2, TaB2 powders or mixtures thereof and of 10% to 35% by volume of SiC. A preferred material is prepared by hot pressing of ZrB2 and SiC powders. These materials have good resistance to oxidation, to thermal stresses, and to ablation, and have good mechanical integrity.
Document [7] relates to composite ceramic compositions which resist to high temperature ablation and which may notably be used for the outer thermal protection of spacecrafts such as space shuttles.
These compositions notably comprise mixtures of zirconium diboride and of zirconium carbide with silicon carbide, mixtures of hafnium diboride and of hafnium carbide with silicon carbide and mixtures of zirconium and hafnium diborides and/or carbides with silicon carbides.
These ceramics are prepared by sintering under pressure a mixture of powders at a temperature which generally ranges from 1,850° C. to 2,250° C.
Different compositions are tested under various heat flows.
The composition consisting of ZrC (20% by volume), ZrB2 (16% by volume) and the remainder being SiC, has under a flux of 400 W/cm2, under a pressure of 0.075 atm and at a temperature of 2,180° C., an ablation rate of 1.97 μm/min. This is the best result obtained in document [7], under a condition for active oxidation of SiC.
Document [8] describes a coating made of refractory carbide for a surface of a carbon substrate, subject to temperature and abrasion stresses, such as nozzle throats, turbine blades, heat shields and hypersonic structures.
The carbon substrate may be made of pyrolytic carbon or made of a carbon-carbon composite.
The coating is prepared by heating the substrate in a furnace in vacuo and by introducing a halide of a metal forming a carbide in the furnace. The metal forms a carbide with the carbon on a first portion of the surface of the substrate. Next, a hydrocarbon gas is added to the halide and a continuous layer of carbide is thereby formed on the first portion.
The preferred carbide is hafnium carbide HfC, but silicon carbide SiC, tantalum carbide TaC, zirconium carbide ZrC, and mixed carbides of silicon, tantalum or zirconium with hafnium are also mentioned.
This carbide layer resists to cracking and to spalling.
It should be noted that according to document [1], the most refractory carbide which is HfC, seems to protect the carbon up to 1,300° C.
Another technique also used for depositing carbide layers is the so-called pack cementation technique.
This technique, which is notably described in document [9], consists of preparing a mixture of a refractory metal (for example chromium) powder, of a metal oxide (for example alumina) and of a catalyst (for example ammonium chloride).
The mixture is then put into contact with the carbon part to be coated, and the assembly formed by the part and by the mixture is brought to a temperature of 1,000° C. under argon. The metal then reacts with the substrate in order to form a carbide layer.
The main drawback of PAOs based on carbides is that several carbides must mandatorily be associated in these PAOs.
Indeed, no carbide can by itself ensure resistance to a high temperature and low oxidation of the substrate.
Other further PAO coatings are multi-layer coatings.
Indeed, generally, a PAO consisting of a single chemical compound is inoperative at a high temperature, i.e. a temperature generally above 1,200° C.
Indeed, in order to overcome the problems of diffusion of oxygen, of compatibility with carbon, and of matching of the coefficients of expansion, it is preferable to produce a multi-layer or multi-sequence PAO.
Document [10] describes a refractory structure capable of prolongedly resisting to temperatures exceeding at least about 2,500° F. in an oxidizing environment which comprises a carbon-carbon composite substrate resistant to high temperatures and a coating resistant to high temperature oxidation (i.e. a PAO) formed in situ on the surface of the substrate and comprising alternating thin layers of SiC and of a carbide of a metal from Group IVB, such as HfC or ZrC. These alternating layers have a thickness from 1 to 10 μm and the total thickness of the PAO is from about 130 to 500 μm.
The layer in contact with the CIC is preferably SiC.
The making of these layers is ensured by chemical vapor deposition (CVD) at a temperature comprised between about 1,090° C. and 1,400° C. (more specifically at a temperature of about 1,198° C.) under a pressure comprised between about 6 mbars and 666 mbars (more specifically under a pressure of about 26 mbars).
The precursor of SiC is methyltrichlorosilane (MTS) and the one of HfC is hafnium tetrachloride obtained by flushing chlorine gas over hafnium metal at 510° C. The alternation of the layers is obtained by opening or cutting off the chlorine supply every 2 minutes.
The authors indicate that such a PAO efficiently protects a C/C for several hours at 1,760° C. For shorter protection times, the latter resists up to 1,930° C.
The preparation of multi-layer coatings is however long, complex and costly, it requires multiple steps and complicated apparatuses.
Besides, hot sintering technology with a pulsed electric field is known (Spark Plasma Sintering or SPS) also known under the name of Field Activated Sintering Technique or FAST.
The first patent applications [11], [12] concerning this technology were filed by K. INOUE at the end of the 60's.
But one had to wait until the end of the 1990s for an exponential increase in the number of patents and publications relating to the SPS technique. In Europe, the number of SPS machines actually increased only at the beginning of the early 2,000 s.
SPS is a sintering technique which consists of simultaneously applying on the solid or powdery sample to be densified, or on the part to be assembled, a uniaxial pressure and current pulses of high intensity which cause a rise in temperature of the sample.
The current is applied as trains of current pulses, for example with a period of 3.2 ms, the intensity of which may reach several thousand amperes, for example up to 8,000 A, or even 50,000 A.
The powders or parts may be made of metal, ceramics or polymers.
The current is applied to the sample via a circuit of graphite plates and pistons, the powder for example is inserted into the inside of a tabletting machine (pelletizer) made of graphite.
The assembly consisting of the tabletting machine, the pistons and the plates is the only circuit in the vacuum chamber for which temperature rises.
More exactly, the principle of the operation of an SPS apparatus and of its main units is illustrated in FIG. 1. The powder (1) is placed in a graphite sleeve (2), between two pistons (3). Pressure (4) is applied to these pistons (3), and a DC current (5) is applied to electrodes (6). The powder (1), the pistons (3), the graphite sleeve (2) and a portion of the electrodes (6) are placed inside a vacuum chamber (7). Instead and in place of the powder, it is possible to place between both pistons two ceramic parts to be assembled so as to have in the matrix the piston-first ceramic-second ceramic-piston succession.
The temperature is tracked via an optical pyrometer which also controls the electric power injected into the assembly. As this was already mentioned above, the currents used during sintering may range up to 50,000 A.
The main benefit of SPS technology is the possibility of densifying the samples in relatively short times of the order of a few minutes, for example from 5 to 10 minutes.
The rapidity of sintering often gives the possibility of minimizing the growth of grains and of attaining for certain materials a density close to 100%.
The use of the SPS for preparing a coating for the PAO has neither been described nor suggested in the prior art as it has been studied above.
Indeed, this study from the prior art mainly discloses three types of methods for preparing a PAO coating, which are:                the methods in which a slurry is prepared and deposited on the substrate, and then conventional sintering is carried out. These methods are notably illustrated by document [4];        the methods in which a mixing of powders is carried out followed by hot compression sintering. These methods are notably illustrated by document [7];        the chemical vapor deposition (CVD) methods.        
Considering the foregoing, there exists a need for a method for preparing a protective coating against oxidation (oxidation-protective coating) on a part made of a material which may be oxidized, this coating being a monolayer coating giving protection against oxidation at high temperatures, for example above 1,200° C.
In other words there exists a need for a method for preparing a monolayer protective coating against oxidation, which is also refractory, or even highly refractory.
In particular, there exists a need for a method for preparing a monolayer coating which ensures effective protection against oxidation, at high temperatures, of parts made of carbon-carbon composite materials.
Further there exists a need for such a method which is simple, reliable, rapid, inexpensive and which gives the possibility of obtaining a dense, quality coating, and as far as possible, free of cracks.
The goal of the invention is to provide a method for preparing a protective coating against oxidation on at least one surface of at least one part made of at least one material which may be oxidized, which i.a. meets this need and which does not have the drawbacks, defects, limitations and disadvantages of the methods of the prior art and which solves the problems of the methods of the prior art.